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University of Arizona


Tom Wilson, director Submillimeter Telescope Observatory


Lori Stiles, University of Arizona


Jim Cornell, Harvard-Smithsonian Center for Astrophysics


SAFFORD, Ariz. — A unique detector of astronomical radiation, developed at the Harvard-Smithsonian Center
for Astrophysics (CfA), in Cambridge, Mass. and tested at the Heinrich Hertz Submillimeter Telescope (HHT) on
Mount Graham, Ariz., has made the first ground-based measurements of radio emission from interstellar
molecules in the “terahertz waveband” — a virtually unexplored part of the astronomical spectrum.

The unique combination of the detector, the excellent high dry site and the accurate telescope were all
necessary for this milestone in radioastronomy, said Tom Wilson , director of the Submillimeter Telescope
Observatory. The SMTO operates the HHT.

The detector is a superconducting hot-electron bolometer (HEB), a device analogous to the familiar AM radio
receiver, but operating at terahertz frequencies, which are about a million times higher than AM radio
frequencies. The key to the HEB is a detector made of a superconducting thin film of niobium nitride, developed
in a collaboration between the CfA’s Submillimeter Receiver Laboratory and a group at the Moscow State
Pedagogical University.

The new receiver is capable of detecting and amplifying very-high-frequency signals with very fine frequency
resolution, so it can detect the spectral lines, or chemical fingerprints, of interstellar molecules which emit radio
signals at terahertz frequencies — the highest frequencies ever achieved with any radio receiver. The
wavelengths corresponding to terahertz frequencies are smaller than one-third of a millimeter, that is, closer to
infrared, about midway between optical emissions and frequencies usually observed by large radio telescopes
such as the VLA at Socorro, N.M.

The receiver was installed on the HHT, a joint project of the Steward Observatory at the University of Arizona
(UA) in Tucson and the
Max-Planck-Institute for Radioastronomy in Bonn, Germany. The high, dry location of the 10-meter-diameter
(33-foot) telescope allows the high-frequency signals to reach the telescope with minimum atmospheric

The HHT’s reflector is a precisely figured surface — accurate to within 12 micrometers, or about one-fifth the
thickness of a human hair. This extreme reflector smoothness and the excellent pointing of the HHT are
essential for terahertz measurements. The very best observing weather is also critical. When that weather
materialized the night of Jan. 7, 2000, Max Planck astronomers interrupted their scheduled telescope
observations so the other astronomers could attempt the terahertz measurements.

Scientists from the CfA and UA groups collaborated in making the observations with the new receiver,
detecting emission from molecules of carbon monoxide (CO) in the Kleinmann-Low Nebula in the Orion Molecular
Cloud. The CO emission indicates that some of the gas in this star-forming cloud is some ten times hotter than

More important, these observations demonstrate that a “terahertz window on the universe” can be opened
for ground-based astronomy. Several groups are now preparing instruments to exploit this new opportunity.

The Heinrich Hertz Telescope scientists involved with the first terahertz observations include Tom Wilson,
Ferdinand Patt, Bill Peters, and Bob Stupak.Christian Henkel and Wilfred Walsh of the Max Planck Institute for
Radioastronomy suspended their regularly scheduled HHT observing time during the critical “best weather”
opportunity to get terahertz signals.

The scientists involved in the first terahertz observations include Ray Blundell, Todd Hunter, Scott Paine, Cosmo
Papa, and Edward Tong at the CfA; Jonathon Kawamura at Caltech; and Eugene Gershenzon and Gregory
Gol’tsman at the Moscow State Pedagogical University.



Ray Blundell, CfA